The present invention relates generally to analytical instruments, and specifically to instruments and methods for thin film analysis using X-rays.
The National Technology Roadmap for Semiconductor (NTRS), published in 1997 by the Semiconductor Industry Association (SIA), indicates that metrology and testing is one of the xe2x80x9cGrand Challengesxe2x80x9d facing the semiconductor industry as it moves toward advanced technologies, such as 0.18 xcexcm and 0.13 xcexcm design rules. In the area of film thickness metrology, the required accuracy when measuring films with thickness less than 3 nm has stretched the capabilities of current film thickness equipment to its physical limits. In addition, the development of new materials, such as low-k dielectrics to accommodate the need for high speed devices and high-k dielectrics as a replacement for silicon dioxide, also require new capabilities for film thickness metrology.
Traditionally, films processed in semiconductor tabs are classed as transparent or opaque, requiring two distinct classes of film thickness metrology systems: optical equipment for thickness measurement of transparent films, and non-optical equipment for opaque films. Non-optical thickness measurement systems include laser acoustic devices, four-point probes, profilometers and X-ray fluorescent spectroscopy equipment.
Optical equipment, such as reflectometers and ellipsometers, have been widely used in thickness measurement and characterization of dielectric and other transparent films in semiconductor fabs. However, as the gate silicon dioxide thickness is reduced to less than 3 nm, the uncertainty in accuracy of these optical technologies, due to the difference in the assumed optical properties between bulk and thin film structures, is practically too large to be used for monitoring and controlling process equipment. For work with high-k and low-k dielectric films, additional film properties, such as composition and/or mass density, must also be known. These needs are not satisfied by available optical equipment.
Resistivity measurement based on four-point probe techniques has been used to deduced the thickness of metal films or other materials that are highly absorptive to optical radiation. Profilometers are also used to measure a step height at the surface of a sample, and the thickness of the top layer on the sample is deduced from the step height. These measurement technique are destructive, due to the contact between the measurement probes and the films under investigation. Furthermore, they are capable of measuring only the top surface layer, and the accuracy of thickness measurement is highly dependent on the condition of the measurement probes. In addition, the resistivity measurement of the four-point probe is highly susceptible to error when the materials under investigation vary in composition or density.
X-ray fluorescent spectroscopy has also been used to measure the thickness, composition and other properties metal and other films. This technique, however, is incapable of analyzing multilayer film structures that contain the same element in more than one layer, and it is ineffective when the elements to be analyzed are of low atomic number (Z). The high-power X-ray radiation used to excite the sample is destructive, and the measurement process is generally time-consuming and requires extensive calibration in order to obtain quantitative results.
Recently, the emergence of laser acoustic technologies has provided the capability of measuring multi-layer metal film structures. In operation, such techniques require layers of metal or low elastic constant materials in order to launch and sense an acoustic wave, which is used to probe the sample under investigation. The fundamental drawback of laser acoustic measurement is its inability to launch or sense an acoustic wave in dielectric films due to the high elastic constant of such films.
X-ray reflectometry (XRR) is a well-known technique for measuring the thickness, electron density and surface quality of thin film layers deposited on a substrate. Conventional X-ray reflectometers are sold by a number of companies, among them Technos (Osaka, Japan), Siemens (Munich, Germany) and Bede Scientific Instrument (Durham, UK). Such reflectometers typically operate by irradiating a sample with a beam of X-rays at grazing incidence, i.e., at a small angle relative to the surface of the sample, near the total external reflection angle of the sample material. Sequential measurement of X-ray intensity reflected from the sample as a function of angle gives a pattern of interference fringes, which is analyzed to determine the properties of the film layers responsible for creating the fringe pattern. The effectiveness of XRR is limited to layers that are less than 200 nm thick and have a surface roughness of no more than about 10 nm.
A method for performing the required analysis to determine film thickness from XRR data is described, for example, in U.S. Pat. No. 5,740,226, to Komiya et al., whose disclosure is incorporated herein by reference. Komiya et al. describe the application of their method to various types of thin films that are used in semiconductor electronic devices, including specifically SiO2, Ti and TiN.
U.S. Pat. No. 5,619,548, to Koppel, whose disclosure is likewise incorporated herein by reference, describes an X-ray thickness gauge based on reflectometric measurement. A curved, reflective X-ray monochromator is used to focus X-rays onto the surface of a sample. A position-sensitive detector, such as a photodiode detector array, senses the X-rays reflected from the surface and produces an intensity signal as a function of reflection angle. The angle-dependent signal is analyzed to determine properties of the structure of a thin film layer on the sample, including thickness, electron density and surface roughness. In order to determine the mass density of the layers, however, prior information is required regarding the composition of the analyzed layer, which cannot be determined based on XRR alone.
U.S. Pat. No. 5,923,720, to Barton et al., whose disclosure is incorporated herein by reference, also describes an X-ray spectrometer based on a curved crystal monochromator. The monochromator has the shape of a tapered logarithmic spiral, which is described as achieving a finer focal spot on a sample surface than prior art monochromators. Barton et al. calculate that the theoretical minimum spot size achievable by their monochromator is 5 nm. X-rays reflected or diffracted from the sample surface are received by a position-sensitive detector. It is suggested that the X-ray spectrometer may be used in surface mapping and film thickness measurements, with application to real-time in situ control of a film deposition system, such as systems used in MOCVD.
U.S. Pat. No. 5,003,569 to Okada et al., whose disclosure is incorporated herein by reference, describes a thickness determination method for organic films based on X-ray reflectometry. The organic film to be measured is irradiated with X-rays at a certain angle of incidence, and the angle is varied in order to find the angle of reflection at which the X-ray intensity reaches a peak. The peak is used to find the thickness of the film.
Another common method of X-ray reflectometric measurement is described, for example, in an article by Naudon et al., entitled xe2x80x9cNew Apparatus for Grazing X-ray Reflectometry in the Angle-Resolved Dispersive Mode,xe2x80x9d in Journal of Applied Crystallography 22 (1989), p. 460, which is incorporated herein by reference. A divergent beam of X-rays is directed toward the surface of a sample at grazing incidence, and a detector opposite the X-ray beam source collects reflected X-rays. A knife edge is placed close to the sample surface immediately above a measurement location in order to cut off the primary X-ray beam, so that only X-rays reflected from the measurement location reach the detector. A monochromator between the sample and the detector (rather than between the source and sample, as in U.S. Pat. No. 5,619,548) selects the wavelength of the reflected X-ray beam that is to reach the detector. The reflected X-ray signal is used, inter alia, to calculate the density of a surface layer of the sample, although in this method, too, a priori knowledge of the composition of the sample is required in order to find the mass density.
An article by Lengeler, entitled xe2x80x9cX-ray Reflection, a New Tool for Investigating Layered Structures and Interfaces,xe2x80x9d in Advances in X-ray Analysis 35 (1992), p. 127, which is incorporated herein by reference, describes a system for measurement of grazing-incidence X-ray reflection, in which X-ray fluorescence is also measured. A sample is irradiated by an X-ray source at grazing incidence. One X-ray detector captures X-rays reflected (likewise at grazing incidence) from the surface of the sample, while another detector above the sample captures X-ray fluorescence emitted by the sample due to excitation by the X-ray source. Analysis of the fluorescence emitted when the sample is excited at an angle below the critical angle for total external reflection of the incident X-rays, as described in this article, is known in the art as total reflection X-ray fluorescence (TXRF) analysis.
Another article, by Leenaers et al., entitled xe2x80x9cApplications of Glancing Incidence X-ray Analysis,xe2x80x9d in X-ray Spectrometry 26 (1997), p. 115, which is incorporated herein by reference, describes a method known as glancing incidence X-ray analysis (GIXA). The method combines X-ray reflectivity and angle-dependent X-ray fluorescence measurements to obtain a structural and chemical analysis of a sample.
An article by Wiener et al., entitled xe2x80x9cCharacterization of Titanium Nitride Layers by Grazing-Emission X-ray Fluorescence Spectrometry,xe2x80x9d in Applied Surface Science 125 (1998), p. 129, which is incorporated herein by reference describes an alternative method for analyzing thin film layers. A sample is irradiated by an X-ray source at normal or near-normal incidence, and fluorescent X-ray photons emitted by the sample are collected at a grazing angle, close to the surface. The spectrum of the collected photons is analyzed by a technique of wavelength dispersion, as is known in the art, and the distribution of photons by emission angle is determined, as well. The resultant data provide information about the thickness and composition of thin film layers on the sample.
Energy dispersion techniques can also be used to analyze the spectral distribution of reflected photons, as described, for example, in a paper by Windover et al., entitled xe2x80x9cThin Film Density Determination by Multiple Radiation Energy Dispersive X-ray Reflectivity,xe2x80x9d presented at the 47th Annual Denver X-ray Conference (August, 1998), which is incorporated herein by reference.
Another paper presented at the August, 1998, Denver X-ray Conference, by Funahashi et al., entitled xe2x80x9cBST Thin Film Evaluation Using X-ray Fluorescence and Reflectivity Method,xe2x80x9d which is incorporated herein by reference, describes further methods that combine XRR and XRF measurements. The paper points out that measuring reflectivity provides structural information, such as thickness and density of thin films, which can be useful in preparing accurate calibration constants for inline XRF measurements. Moreover, the refractive indices of the thin film, which are needed for accurately fitting the interference pattern of the reflected signal to an optical model so as to find the film layer thickness, can be calculated based on the composition of the film material as determined by XRF. A paper by Remmel et al., entitled xe2x80x9cDevelopment of an XRF Metrology Method for Composition and Thickness of Barium Strontium Titanate Thin Films,xe2x80x9d also presented at the August, 1998, Denver X-ray Conference and incorporated herein by reference, describes methods similar to those of Funahashi et al., in which XRF is combined with film thickness measurements based on spectroscopic ellipsometry and X-ray reflectivity.
It is an object of the present invention to provide improved methods and apparatus for analysis of the properties of surface layers of a sample.
It is a further object of some aspects of the present invention to provide improved methods and apparatus for thin film evaluation, particularly for use in the production of integrated circuits on semiconductor wafers.
It is yet a further object of some aspects of the present invention to provide methods and apparatus for X-ray microanalysis of features at the surface of a sample.
In preferred embodiments of the present invention, an X-ray microanalyzer comprises at least one X-ray source and at least two X-ray detector devices. Most preferably, the detector devices comprise detector arrays. The X-ray source emits X-rays, which are focused to a fine spot on the surface of a sample under analysis, causing emission of fluorescent X-rays photons from the spot. Typically the sample comprises a semiconductor wafer on which one or more thin film layers have been deposited in a predetermined pattern. Preferably, the X-rays are focused by a monolithic capillary array, most preferably as described in U.S. patent application Ser. No. 09/114,789, which is assigned to the assignee of the present patent application and incorporated herein by reference. The capillary array focuses the X-rays to a spot that is less than 500 xcexcm in diameter, more preferably less than 100 xcexcm in diameter, and most preferably approximately 50 xcexcm in diameter, so that the focused X-ray irradiation can be substantially confined to the area of a selected feature on the wafer, such as a scribe line or an element of an integrated circuit.
The detector arrays comprise a first array, which is positioned opposite the sample to capture X-ray fluorescence emitted therefrom at relatively high angles to the surface, and a second array, which is positioned so as to capture X-rays at a relatively low angle, preferably a grazing angle with respect to the surface. The high-angle fluorescent X-rays captured by the first array are analyzed, primarily for the purpose of determining the elemental composition and/or thickness of the sample within the feature on which the X-ray spot is focused. The low-angle X-rays captured by the second array are analyzed for the purpose of determining the thickness and density and roughness of one or more thin film layers at the surface of the sample. The results of the analysis of the X-rays captured by the two arrays are combined to derive a complete, calibrated analysis of the selected feature, preferably including the composition, thickness and density of one or more thin film layers within the feature.
Thus, while X-ray analyzers known in the art can give only a gross picture, averaged over relatively large areas of a wafer or other sample, microanalyzers in accordance with preferred embodiments of the present invention are capable of analyzing surface layer properties in a small, precisely-selected spot. The present invention is thus particularly useful in non-destructive analysis of the properties of elements of integrated circuits on a semiconductor wafer, both in various stages of production and in post-production testing. It enables the mass density of thin film layers to be measured directly, in a single measurement operation, without a priori knowledge of the film composition.
In some preferred embodiments of the present invention, the X-ray photons captured by the second detector array are fluorescent photons emitted by the sample at low angles, so that only a single X-ray source provides excitation for both high-angle and grazing-angle emission measurements. In other preferred embodiments, however, a second X-ray source is positioned so as to irradiate the sample at grazing incidence, and the second detector array is used to capture X-rays reflected from the sample surface. The use of reflectometry, rather than low-angle fluorescence measurement, provides a higher signal flux and therefore faster measurement throughput. The beam of the second X-ray source is preferably monochromatized either before or after impinging on the sample, as is known in the art. Most preferably, the beam is focused to a fine spot describing a generally oblong shape on the sample, wherein the spot has a transverse dimension on the order of the focused spot size created by the X-ray source used for fluorescence excitation.
There is therefore provided, in accordance with a preferred embodiment of the present invention, apparatus for X-ray microanalysis of a sample, including:
an X-ray source, which irradiates a spot having a dimension less than 500 xcexcm on a surface of the sample;
a first X-ray detector, which captures fluorescent X-rays emitted from the sample, responsive to the irradiation, at a high angle relative to the surface of the sample;
a second X-ray detector, which captures X-rays from the spot at a grazing angle relative to the surface of the sample; and
processing circuitry, which receives respective signals from the first and second X-ray detectors responsive to the X-rays captured thereby, and which analyzes the signals in combination to determine a property of a surface layer of the sample within the area of the spot.
Preferably, the spot has a dimension less than 100 m. Most preferably, the spot has a dimension of about 50 xcexcm or less.
Preferably, the first and second X-ray detectors include detector arrays, wherein the first detector array includes a plurality of detector elements disposed around a beam axis of the X-ray source. Preferably, the second detector captures fluorescent X-rays emitted from the sample at the grazing angle.
In a preferred embodiment, the first detector array captures fluorescent X-rays emitted from the sample when the X-ray source irradiates the sample at an angle below a total external reflection angle of the sample.
Preferably, the X-ray source includes a first X-ray source, which irradiates the spot along an axis generally perpendicular to the surface of the sample, and including a second X-ray source, which irradiates an area of the surface including the spot at the grazing angle, so that the second X-ray detector captures reflected X-rays from the second source. Most preferably, the area irradiated by the second X-ray source has a transverse dimension approximately equal to the dimension of the spot irradiated by the first X-ray source. Preferably, the apparatus includes a monochromator, which monochromatizes X-rays from the second X-ray source.
There is further provided, in accordance with a preferred embodiment of the present invention, apparatus for X-ray microanalysis of integrated circuits produced on a semiconductor wafer, including:
an X-ray source, which irradiates an area on a surface of the wafer corresponding to a selected feature of the integrated circuits;
a first X-ray detector, which captures fluorescent X-rays emitted from the irradiated area, responsive to the irradiation, at a high angle relative to the surface of the wafer;
a second X-ray detector, which captures X-rays from the irradiated area at a grazing angle relative to the surface of the wafer; and
processing circuitry, which receives respective signals from the first and second X-ray detectors responsive to the X-rays captured thereby, and which analyzes the signals in combination to determine a property of the selected feature of the integrated circuits.
Preferably, the integrated circuits are produced by depositing a thin film layer on the wafer, and wherein the property of the selected feature includes a thin film property, most preferably including a composition of the film, a thickness of the film and/or a density of the film. In a preferred embodiment, multiple thin film layers are deposited on the wafer, and wherein the processing circuitry determines properties of two or more of the multiple layers.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for X-ray microanalysis of a sample, including:
irradiating a spot having a dimension less than 500 xcexcm on a surface of the sample;
capturing fluorescent X-rays emitted from the sample, responsive to the irradiation, at a high angle relative to the surface of the sample;
capturing grazing angle X-rays from the spot at a grazing angle relative to the surface of the sample; and
receiving respective signals from the first and second X-ray detectors responsive to the X-rays captured thereby; and
analyzing the signals in combination to determine a property of a surface layer of the sample within the area of the spot.
Preferably, the sample includes a semiconductor wafer, and irradiating the spot includes selectively irradiating a particular feature of integrated circuits produced on the wafer, wherein analyzing the signals includes determining a property of a thin film on the wafer used in producing the integrated circuits.
In preferred embodiments, multiple thin film layers are deposited on the wafer, and determining the property of the thin film includes determining properties of two or more of the multiple layers. In one such preferred embodiment, the multiple layers include a copper layer, a tantalum layer and a tantalum nitride layer, wherein the multiple layers include an intermediate layer dielectric. In another preferred embodiment, the thin film has a thickness substantially greater than 200 nm.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: